As the fibrous carbon material, a carbon nanotube (CNT) and a vapor growth carbon fiber (VGCF) are well known. Both the carbon nanotube and the vapor growth carbon fiber are fine tube form structures constituted with graphene, and are differentiated by the difference in the lamination structure and the fiber diameter associated therewith, as will be described below.
Graphene is a net of honeycomb structure in which six carbon atoms are regularly arranged in a two-dimensional manner, and is also referred to as a carbon hexagonal net plane. The substance in which this graphene is laminated with regularity is referred to as graphite. A single-layer or multiple-layer fine tube form structures constituted with this graphene is a fibrous carbon material, and includes both a carbon nanotube and a vapor growth carbon fiber.
That is, the carbon nanotube is a seamless tube in which graphene is rounded in a tubular shape, and may be a single-layer one or a multiple-layer one in which the layers are concentrically laminated. The single-layer one is referred to also as a single-layer nanotube, and the multiple-layer one is referred to also as a multiple-layer nanotube.
Also, the vapor growth carbon fiber is one having, in a core part, a graphene tube of a single layer or plural layers in which graphene is rounded in a tubular shape, namely a carbon nanotube, where graphite is laminated in a radial direction of the graphene tube so as to surround the core part in a multiple manner and in a polygonal shape, and is referred to also as a super multiple-layer carbon nanotube because of its structure.
In other words, the single-layer or multiple-layer carbon tube that is present at the central part of a vapor growth carbon fiber is a carbon nanotube.
Various composite materials are proposed in which it is intended to improve the heat conductivity and the electric conductivity by a fibrous carbon material while taking advantage of the features of a metal or ceramics by allowing such a fibrous carbon material to be contained in a metal, ceramics, or a mixture thereof. A representative composite material is a composite material in which carbon nanotubes are compounded into a substrate made from a metal, ceramics, or a mixture thereof.
However, the fibrous carbon material is essentially short and is not continuous in one direction. Therefore, even if it could be oriented in one direction, it would be difficult to improve the heat conductivity to a great extent. That is, the carbon nanotube is thin and short. Typically, the carbon nanotube is present by being entangled in a cotton form. Recently, some have appeared that have a high straightness and can be strongly oriented in one direction. On the other hand, the vapor growth carbon fiber is comparatively thick and long, and has a good straightness, so that it can be relatively easily oriented in one direction, whereby the heat conductivity can be improved more easily as compared with the carbon nanotube. However, even with that vapor growth carbon fiber, a large amount of vapor growth carbon fiber is needed to improve the heat conductivity to a great extent, thereby inviting aggravation of economical property.
In the meantime, the length of a carbon nanotube is several hundred μm and the thickness is 100 nm or less, whereas the length of a vapor growth carbon fiber is at most 2 to 5 cm and the thickness is 500 nm to 100 μm.
Under these circumstances, the present inventors are continuing to make researches on a structure capable of effectively utilizing the electric conduction property, the heat conductivity, and the strength property of a fibrous carbon material in a composite material in which a fibrous carbon material such as a carbon nanotube is compounded in a substrate, based on commission of development by the Independent Administrative Agency, Japan Science and Technology Agency. During this process, the present inventors have previously developed a composite material having extremely high heat conductivity in which fibrous carbon material layers are laminated at a predetermined interval in a spark plasma sintered body of an aluminum powder (see Patent Document 1).    Patent Document 1: International Publication WO2006/120803 pamphlet
This composite material is, for example, a cylindrical body. In the case of a cylindrical body, spark plasma sintered body layers of an aluminum powder and carbon fiber sheets in which fibrous carbon materials are oriented in a specific direction perpendicular to the central line are alternately laminated in a central line direction thereof, whereby it is particularly excellent in heat conductivity in the orientation direction of the fibrous carbon material in the carbon fiber sheet (the direction perpendicular to the central line in the case of a cylindrical body).
As the fibrous carbon material used herein, a thick and long vapor growth carbon fiber is advantageous. A thick and long vapor growth carbon fiber also has a high orientation property. The heat conductivity rises to about 300 W/mK when the vapor growth carbon fiber is contained at 30 wt %, while the heat conductivity of an aluminum powder sintered body serving as a substrate is about 200 W/mK. That is, the heat conductivity increases by about 1.5-fold when the vapor growth carbon fiber is contained at 30 wt %. Also, the heat conductivity rises to 500 W/mK, which is about 2.5-fold of that of the substrate, when the vapor growth carbon fiber is contained at 60 wt %.
However, even with a vapor growth carbon fiber, a fibrous carbon material is essentially expensive. A carbon nanotube has a low orientation property because of being thin and short, and the price is further higher. A long one having a high straightness property is developed; however, a long one is especially expensive. Due to these circumstances, development of an economical high heat conduction composite material that can reduce the amount of use of the fibrous carbon material as much as possible while maintaining high heat conductivity is waited for.